1. Field of the Invention
The present invention relates to a method for correcting jitter of transmission data. In particular, the present invention relates to a method for correcting jitter of transmission data that arises from a clock frequency difference between devices operating with independent clocks when sound or music data that requires real-time characteristics is transmitted between such devices for playback of the data.
2. Description of the Related Art
Information, including sound and music data, may be transmitted and received between devices through a wireless connection such as BLUETOOTH or wireless LAN. A variety of audio and video systems are proposed in which a DVD player or a personal computer that plays back and displays video images, a CD players an FM tuner or similar source devices are wirelessly connected to a speaker which outputs sound signals from such devices received by the wireless connection.
FIG. 7 shows an example of an audio system according to the known art, illustrating how sound data is transmitted and played back.
The audio system according to the known art shown in FIG. 7 includes a transmitter 41 (source device) that plays back or generates sound or music data (hereinafter, both sound and music data together or separately may be referred to as music data), such as a DVD player, a personal computer, a CD player, or an FM tuner; and a receiver 42 having a digital-to-analog (D/A) converter (not shown), an audio amplifier (AMP) (not shown), and a speaker (not shown) connected thereto. The receiver 42 includes a data buffer 43 for temporarily storing data. The receiver 42 receives music data in packets from the transmitter 41 into the data buffer 43, transfers the music data from the data buffer 43 to the digital-to-analog converter, and finally outputs the data from the speaker.
In the known audio system described above, the transmitter 41 and the receiver 42 are operated based on their respective reference clocks, which are independent of each other. Let us assume that the transmitter 41 has a reference clock of nominally 44.1 kHz, which is the sampling frequency of music data, and breaks music data into packets to transmit the data to the receiver 42 according to the 44.1-kHz reference clock. Under this assumption, the receiver 42 receives the packets of music data from the transmitter 41 and sequentially stores the received music data into the data buffer 43 according to the reference clock of the transmitter 41. The data buffer 43 operates on a FIFO scheme and can normally store about 200 ms of data.
The receiver 42 reads out the music data stored in the data buffer 43 according to its 44.1-kHz reference clock and transfers the music data to the digital-to-analog converter. The music signals are converted to analog signals through the digital-to-analog converter, amplified through a device such as the audio amplifier and are then output from the speaker.
For example, Japanese Unexamined Patent Application Publication No. 2001-128171 discloses a technology that allows sound and music to be transmitted wirelessly between devices as described above.
In the known method for transmitting data, the transmitter 41 and the receiver 42 are provided with respective reference clocks, which are independent of each other. The nominal reference clock frequency of the receiver 42, may not completely match that of the transmitter 41. The data buffer 43 in the receiver 42 is used to accommodate this difference between the nominal frequencies of the reference clocks.
The known art described above, however, has a disadvantage in that while music data is being read from the data buffer 43 for continuous playback, the data buffer 43 may become empty or full due to the difference in reference clock frequencies, thus generating jitter that causes unpleasant sound artifacts during playback, such as skipping of the music data being played back, or other sounds not faithfully reproducing the data that has been received over the wireless connection.